One commonly known example of a bearing for a rotating table of a machine tool or the like is a composite-type cylindrical roll bearing that can bear loads in both the radial and thrust directions. As shown in FIG. 5, a composite cylindrical roll bearing 100 has an annular outer ring 102 and an annular inner ring 103 having a groove-shaped cross section, and the inner ring comprises first, second, and third receiving surfaces 103a, 103b, 103c that face the inner peripheral portions 102a, 102b at both ends of the outer ring 102 and that also face a circular inner peripheral surface 102c of the outer ring at specified intervals. Thrust bearing rollers 104 for bearing thrust loads are located between the inner peripheral portion 102a at one end of the outer ring 102 and the first receiving surface 103a of the inner ring 103, and also between the inner peripheral portion 102b at the other end of the outer ring 102 and the second receiving surface 103b of the inner ring 103. Radial bearing rollers 105 for bearing radial loads are located between the circular inner peripheral surface 102c of the outer ring 102 and the third receiving surface 103c of the inner ring 103.
The inner ring 103 comprises a main portion 110 on which the second and third receiving surfaces 103b, 103c are formed, and an annular plate member 111 on which the first receiving surface 103a is formed. The plate member 111 is fixed in place on the main portion 110 by a bolt 112.
The thrust bearing rollers 104 are held by retainers 107 in a rollable state between the inner peripheral portion 102a at one end of the outer ring 102 and the first receiving surface 103a of the inner ring 103, and also between the inner peripheral portion 102b at the other end of the outer ring 102 and the second receiving surface 103b of the inner ring 103. The radial bearing rollers 105 are held in a rollable state between the circular inner peripheral surface 102c of the outer ring 102 and the third receiving surface 103c of the inner ring 103. The rollers are held by annular concavities 109 that have rectangular cross sections and are formed in the third receiving surface 103c of the inner ring 103. Also, the side portions on both sides of the annular concavities 109 allow for positioning in the thrust direction (in the direction of the center line L of the bearing 100).
In the composite cylindrical roll bearing 100 thus configured, the radial bearing rollers 105 are held by the annular concavities 109 formed in the inner ring 103. Therefore, when the inner ring 103 rotates, the radial bearing rollers 105 rotate while revolving around the annular concavities 109. This leads to problems in that sliding frictional force arises between the rollers 105 and the annular concavities 109, and a large amount of frictional force acts on the rollers 105.
Also, since the bottom surfaces and both side surfaces of the annular concavities 109 formed in the third receiving surface 103c of the inner ring 103 are race surfaces in which the rollers 105 move, these surfaces must be formed with a high degree of precision. It is difficult to machine such annular concavities 109 with a high degree of precision compared to cases in which a circular outer peripheral surface or a circular inner peripheral surface is machined into a race surface. If the annular concavities 109 cannot be finished with a high degree of precision, then problems are created in that highly precise positioning is not possible in cases in which the composite cylindrical roll bearing 100 is incorporated into a rotating table of a machine tool or the like.